1. Field of the Invention
This invention relates to exhaust gas driven turbosupercharger systems for internal combustion engines of either the otto cycle or diesel cycle type, and particularly to a turbosupercharger system for obtaining increased power from internal combustion engines by increasing the pressure of inlet air supplied to the engine. The invention contemplates the use of selected engine parameters for simultaneously varying the geometry of both the compressor discharge diffuser housing and the turbine inlet nozzle housing in order to, in effect, provide a turbosupercharger having a size matched to varying engine operating conditions.
2. Description of the Prior Art
The use of turbosuperchargers for compressing the air, or air-fuel mixture, to increase the brake horsepower output of internal combustion engines, has been known and practiced for many years. Turbosupercharging is advantageous since a naturally aspirated or unsupercharged engine is able to draw into the cylinders on suction strokes only from 70-85% of the fuel charge which it is theoretically capable of inducing. Consequently, the mean effective pressures are smaller than they might be, and the power output per in..sup.3 of piston displacement does not reach its maximum value. Since many engines were not structurally designed to withstand stresses created by cylinder pressures above atmospheric, turbosupercharging found its main utility in aircraft about the time of World War II where the turbosupercharger maintained the power of the engine up near its rated output at high altitudes.
Until recently sea-level gasoline engines have not often been turbosupercharged because they are most often of small capacities, and by operating at relatively high rotative speeds engine size may be kept small. Turbosupercharging these engines may create detonation problems without the use of special high-octane fuels. Diesels, however, are frequently turbosupercharged, permitting the burning of larger amounts of fuel without creating excessive combustion chamber temperatures.
The turbosupercharger is exhaust gas turbine-driven and consists basically of a single-stage turbine wheel directly connected to a compressor impeller. The engine exhaust is collected and led to the turbine inlet nozzle where it arrives at a pressure of several lbs./in..sup.2. Excess pressure ratio at high engine RPM across the nozzle of the turbosupercharger is typically controlled by releasing part of the exhaust to the atmosphere through a waste gate. A primary advantage of this type of turbosupercharging is that a part of the energy in the incompletely expanded exhaust gases is utilized rather than wasted. Gas expanding through the turbine nozzles to atmospheric pressure flows across the turbine blades, turning the turbine wheel. The compressor impeller, directly connected to the turbine, draws in air from the intake duct or carburetor, and adiabatically boosts it in pressure and temperature. Fuel is added either before or after pressurization of the air depending on the cycle. While the power necessary to drive the compressor is high, it is more than repaid by increased engine output. The compressor power required is a function of the pressure ratio, air temperature, and compressor efficiency.
In the past, turbosuperchargers were used with internal combustion engines for the explicit purpose of increasing power at high engine RPM, and were essentially useless and often disengaged at low engine RPM. Recently, due primarily to the necessity of conserving energy, turbosuperchargers have been designed for automotive gasoline engines which are advertised to perform smoothly through a broad range of driving conditions. These turbosuperchargers use broad flow rate compressors and a relatively small turbine inlet to provide some boost in power at low engine speeds. The turbosuperchargers are generally mounted between the carburetor and the intake manifold to minimize the inertial lag of the airflow.
A problem with turbosuperchargers of a fixed size is that they inherently cannot provide efficient boost over the entire range of internal combustion engine torque requirements. With fixed geometry compressors and turbines, the turbosupercharger will perform at its maximum efficiency at its design point, and will become less efficient to some degree as engine operation deviates from the design point. While somewhat of an oversimplification, a turbosupercharger designed to boost power at high RPM and volume flows will be large and will not perform efficiently at low RPM and volume flows, while a turbosupercharger designed to perform efficiently at low RPM and volume flows will be small and inadequate at high RPM and volume flows.
The usual solution is an intermediate sized turbosupercharger which has the power wasteage of an open wastegate valve at high RPM, and is too large to give much boost at low RPM, but which is relatively efficient for intermediate engine power conditions.
A solution to this dilemma is a variable geometry turbosupercharger, that is, one in which the effective size varies with engine airflow and engine torque requirements. A turbosupercharger in which the major active components, the compressor and tubine, can be varied with changing engine demands will provide the most efficient operation possible over the full engine operating range. A slight weight penalty and complexity of operation will result, but these undesirable results will be more than overriden by the fuel economy of greater efficiency. In effect, a variable geometry turbosupercharger will act like a small turbosupercharger at low engine power and RPM, and like a large turbosupercharger at high engine power and RPM. The variable geometry components of the disclosed turbosupercharger are not actually the compressor and turbine wheels, but rather the compressor discharge diffuser and the turbine inlet nozzle. Varying the geometry of these elements will effectively create a turbosupercharger which acts, over a range of operation, like a fully variable device.
It is known in the art to vary the inlet housing of a turbine in order to vary turbine flow versus pressure drop characteristics. The turbine inlet housing may be varied in cross-sectional flow area by moving a wall of the turbine nozzle housing, or by rotating the angle of nozzle vanes in the inlet housing if the housing is of the vaned type.
Although less well known than variable turbine inlet housings, it is also known to vary the discharge housing of a radial compressor to regulate its flow versus pressure rise characteristics. A compressor outlet diffuser housing may be varied in cross-sectional flow area in a manner similar to turbines, i.e., by moving a wall of the compressor outlet diffuser housing, or by rotating the angle of the diffuser vanes if the outlet housing is of the vaned type. Moving the vanes at the inlet of a radial compressor, as performed by the prior art for varying compressor geometry, rather than moving the outlet vanes, is not pertinent to the present invention since modulation of inlet vanes cannot change the flow characteristics of the compressor without adversely reducing its pressure rise characteristic. In other words, the prior art modulation of compressor inlet vanes drops pressure output at the same time that it reduces flow. Reduced pressure output destroys the turbosupercharging performance.
Turbosuperchargers for internal combustion engines in the past have utilized variable geometry turbine inlet housings with a variety of control schemes. Some turbosuperchargers have also utilized variable geometry compressor inlet housings, which as described above do not accomplish the object of the present invention. Because of the lack of effective compressor geometry change schemes used in the past, a practical fully-variable geometry turbosupercharger system has not been available prior to the present invention.
While not analogous to internal combustion engine or turbosupercharger technology, an air cycle environmental control system providing cooling air to aircraft enclosures has been designed incorporating movable turbine nozzles and movable compressor diffuser vanes. When implemented in a bootstrap air cycle machine, improved efficiencies over airflow ranges of 4:1 were achieved, twice that achievable with prior air cycle machines. Details of the system may be found in ASME Report #77 ENAS-7, July 11, 1977 in an article entitled "Variable Geometry Air Cycle Machine" by J. Tseka and G. C. Letton, Jr.
It is therefore an object of this invention to provide a variable geometry turbosupercharging system for an internal combustion engine which is highly efficient and provides significantly more engine output torque on demand over a wider range of engine speeds than prior art internal combustion engine turbosupercharging systems.
Another object of this invention is to provide a variable geometry turbosupercharging system for an internal combustion engine in which the compressor discharge diffuser and turbine inlet nozzle are simultaneously varied in area as a function of engine requirements to produce an efficient match between the engine and the turbosupercharger.
A further object of this invention is to provide a variable geometry turbosupercharging system for an internal combustion engine which is mechanically simple and in which the variable geometry function is produced by mechanically linking both the variable turbine inlet geometry and the variable compressor outlet diffuser geometry to a single actuator.
Another object of this invention is a control for varying the geometry of a turbosupercharger turbine inlet and compressor outlet housings over the full range of operating conditions of an internal combustion engine to provide the optimum match between the pressure and volume flow output of the compressor and the pressure and volume flow input requirements of the internal combustion engine.
A further object of this invention is a variable geometry turbosupercharger for internal combustion engines in which the turbosupercharger may be effectively removed from operation during periods of low torque demand by the engine, and in which turbine inlet pressure is controlled to prevent excessive compressor discharge pressure during periods of high torque demand.